1. Field of the Invention
The present invention relates to an improved automatic gear shifting system having automatic and manual modes of operation for use on derailleur equipped bicycles and more particularly, to an improved derailleur shifter where, in the automatic mode of operation the gear ratio between the driving and driven gears can automatically varied in response to wheel speed and crank speed sensors, the output signals therefrom and signals from an operator's control panel being processed by a microcontroller, and where, in the manual mode of operation, the bicycle user can change the gear ratio by activating a pushbutton on the control panel.
2. Description of the Prior
Bicycle transmissions wherein the gear ratio can be automatically varied through a contractible and expandable large diameter sprocket wheel construction in response to the effort exerted upon the pedals by the cyclist are known. For example, in U.S. Pat. No. 4,693,700 to Chappell, a number of such prior art devices are disclosed, including those of varying the gear ratio by applying special constructions at o about the cluster gears to function the derailleur for gear change purposes. The Chappell patent further notes that the prior art constructions included electrical changers, pneumatic changers and other mechanical devices which were intended to remotely operate the derailleur and notes that most or all of the prior art gear changing devices required conscious effort on the part of the bicycle rider in order to effect a desired gear change. The Chappell device comprises a fixed frame connected arm, a shifter arm moveable relative to the fixed arm, the moveable shifter arm carrying a chain follower in sliding engagement with the bicycle chain, a spring biasing means between the two arms urging the follower to depress the chain and a cable interconnected between the moveable shifter arm and the derailleur to automatically shift the derailleur in response to chain tension as sensed by the follower. Thus, indexing of the derailleur to move the chain between the cluster gears is automatically accomplished without the conscious effort on part of the rider.
One obvious disadvantage of the system is that the effected gear ratio is dependent on the stress applied to the bicycle chain, a parameter obviously dependent heavily on the physical efforts of the rider.
U.S. Pat. No. 4,599,079 also to Chappell, discloses a similar device to that set forth in U.S. Pat. No. 4,693,700 and provides a different bicycle chain tension sensing mechanism.
U.S. Pat. No. 4,857,035 to Anderson discloses an automatic bicycle power transmission system, the bicycle having only a single sprocket mounted at the rear wheel without a derailleur system.
U.S. Pat. No. 4,938,324 to Van Dyke discloses an automatic derailleur shifter which improves upon the prior mentioned Chappell patents by providing a hydraulic cylinder stabilizing means to control and slow the pivotal movement of the moveable shifter arm.
While the Chappell and Van Dyke patents disclose an automatic derailleur type shifting mechanism, the systems require a complicated and expensive array of mechanical parts. Further, shifting is dependent upon the tension of the bicycle chain, which, in itself, provides an inefficient way for controlling the derailleur. In this regard, neither the velocity of the bicycle or the rotational speed of the pedals is utilized directly to control the bicycle gear ratio, two parameters which more closely relate to the bicycle rider and which, would prove to be an effective way of providing the most efficient gear ratio for a bicycle rider.